Passive moving object detection system and method using signals transmitted by a mobile telephone station

ABSTRACT

A passive object detection system ( 1 ) comprises first and second antennas ( 4, 6 ) and a processor ( 8 ). The first antenna ( 4 ) is adapted to receive a signal transmitted by a mobile telephone base station; the second antenna ( 6 ) is adapted to receive the signal transmitted by a mobile telephone base station ( 2 ) after it has been reflected off an object ( 3 ) and the processor compares the signal received from the mobile telephone base station with the signal reflected from the object to derive speed or position information relating to the object therefrom.

This invention relates to a system and method for object detection, inparticular for detecting moving objects.

There are many situations in which it is desirable to detect movingobjects and to provide position and/or speed information about thoseobjects, for example, to detect aircraft, monitor traffic flow or todetect vehicles exceeding the speed limit. Active radar systems areoften used for these purposes, however this requires RF transmissions.Use of RF transmissions has some disadvantages. The transmissions can bedetected by simple receivers, which gives away the presence of thesensor (e.g. car radar detectors used by speeding motorists). Also,there are many legal restrictions on the transmission of radiofrequencies, which may differ from one country to another, so a systemsuitable for use in one country may be illegal in another.

Passive systems using television transmitters have been proposed,however these have certain disadvantages. Although they use high powertransmitters, the objects being detected are often far away and thereceived signal power may be weak.

U.S. Pat. No. 5,604,503 describes a method of separating multipathsignals received at an antenna using a signal preprocessor which permitscoherent bistatic radar detection with a single omnidirectional antenna.

U.S. Pat. No. 6,011,515 describes a vehicle traffic sensor using aroadside antenna to detect signals from a transmitter and signalsreflected off a vehicle.

In accordance with a first aspect of the present invention, a passivemethod of detecting an object comprises receiving a first signaltransmitted by a mobile phone base station, receiving a second signalcomprising the first signal transmitted by the mobile phone base stationafter it has been reflected off an object; and comparing the first andsecond signals to derive data relating to position or speed of movementof the object; the method further comprising providing a plurality ofmobile phone base stations which transmit a signal, such that as theobject moves out of range of one base station, it comes into range foranother, whereby the distance of the object from a base station beingused to determine a position of the object remains substantiallyconstant and the distance of the object from the receiver changes,thereby improving the received power and range.

In accordance with a second aspect of the present invention a passiveobject detection system comprises first and second antennas; andprocessing means; wherein the first antenna is adapted to receive asignal transmitted by a mobile telephone base station; wherein thesecond antenna is adapted to receive the signal transmitted by themobile telephone base station after the signal has been reflected off anobject and wherein the processing means compares the signal receivedfrom the mobile telephone base station with the signal reflected fromthe object and derives speed or position information relating to theobject therefrom; wherein the object is a moving object; and the systemfurther comprises a plurality of mobile phone base stations whichtransmit a signal, such that as the object moves out of range of onebase station, it comes into range for another, whereby the distance ofthe object from a base station being used to determine a position of theobject remains substantially constant and the distance of the objectfrom the receiver changes, thereby improving the received power andrange.

The present invention covers a passive electronic system which makes useof the radio emissions from mobile phone base stations and in particularthe reflection of those radio waves from objects, such as cars, peopleand animals, to detect the location and, if moving, the velocities ofthose objects. No transmissions from the system are required and thedetection system can be carried from place to place and used inconjunction with an existing mobile phone base station nearby. It isparticularly beneficial to police forces enforcing speed limits, thatthe vehicle cannot detect the existence of the sensor. The proliferationof mobile phone bases stations in towns and on main roads gives goodcoverage, in the areas required.

An example of a passive object detection system and method according tothe present invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a plan view of a prior art system;

FIG. 2 is a schematic diagram of one example of a passive objectdetection system according to the invention;

FIG. 3 is a plan view illustrating operation of the system of FIG. 2

FIG. 4 illustrates a sensor of the system of FIG. 2 in more detail;

FIG. 5 illustrates alternative arrangements for the sensor of FIG. 2;and

FIG. 6 is a flow diagram illustrating an algorithm for use in the systemof FIG. 2.

FIG. 1 shows in plan view how a prior art system for detecting objects,in this case using a television transmitter, operates. A tv transmitter20 emits a signal which travels a distance R₁ and is reflected off anobject 21. The reflected signal travels a distance R₂ and is received ata receiver 22. The power P_(R) of the signal received at the receiver 22can be calculated from the equation:$P_{R} = {\frac{P_{T}G_{1}G_{2}\sigma}{\left( {4\pi} \right)^{3}R_{1}^{2}R_{2}^{2}}\quad\lambda^{2}}$where

-   -   P_(T) is is the transmitted power    -   G₁ is the gain of the first antenna    -   G₂ is the gain of the second antenna    -   σ is the radar cross section of the object of interest    -   λ is the wavelength of the transmitted signal    -   R₁ is the distance between the transmitter and the object    -   R₂ is the distance between the receiver and the object        In the prior art system using a tv transmitter, when the object        is moving away from the transmitter the distances R₁ and R₂        increase at a similar rate, so one can assume that the received        power P_(R) is proportional to 1/R⁴, i.e. as the object moves        away there is a fast and significant reduction in received        power.

FIG. 2 illustrates how a passive object detection system 1 according tothe present invention is used in conjunction with radio wavestransmitted by a mobile phone base station 2 and reflected off an object3. In this example, the object in question is a vehicle, but otherobjects could be sensed equally well. The object may be moving orstationary. The system comprises a first antenna 4 which points directlyat the base station 2 and detects radio waves 5 which have travelledalong the shortest path. A second antenna 6 points in the direction ofthe object of interest and detects radio waves 7 reflected off thatobject 3. The antennas may be of any suitable type, such as Yagi orphased arrays. A processor 8 analyses the signals received by the firstand second antennas 4, 6 and compares the phase and frequencies of thesetwo received signals. This is illustrated in more detail in FIG. 4. Theprocessor also measures the time delay between the two received signals.The signals may then be displayed or stored.

A display device 9 takes the output of the processor and displays theinformation derived about frequency difference and time delay betweenthe two signals. The display may convert the information into distanceoff and speed of the object, if it is moving. A recording mechanism mayalso be provided. Such a system would be particularly useful inmonitoring traffic flow.

The advantages of the present invention can be seen from FIG. 3 which isa plan view of the system in operation. As in the prior art, there is areceiver 22, however the present invention does not rely on a singletransmitter. Instead, it takes advantage of the proliferation of mobilephone transmitter aerials 23. This has the effect that instead of theobject moving out of range of the transmitter and so increasing thedistance R₁, when the object moves out of range of one transmitter, itcomes into range for another, so that the distance R₁ remainssubtantially constant whilst R₂ changes. From this, P_(R) can be takento be proportional to 1/R², thereby significantly increasing thereceived power. By using transmitters further away from the receiver,the range of the device is increased over prior art systems because thetarget is always close to a transmitter, despite mobile phonetransmitters operating at higher frequency and lower power than tvtransmitters.

FIG. 4 illustrates the signal processing in more detail. The signalreceived by each antenna 4, 6 is amplified in respective pre-amplifiers10, 11, then converted to a digital signal by analogue to digitalconverters (ADC's) 12, 13. In practice the received signals wouldprobably be mixed down to a lower frequency before being sampled by theADC, for example as shown in FIG. 3.

The output signals from the ADC's 12, 13 are fed into the processor 8,which may be a digital signal processor or some hardware implementationof the algorithm such as an FPGA, EPLD, ASIC or similar. The processoris set up to run an algorithm as illustrated in the flow diagram of FIG.6. This algorithm has two separate functions. Firstly, the algorithmwill determine the delay difference between the two signals, andsecondly it will determine any Doppler shift due to motion of thetarget.

To determine the time delay the algorithm performs a cross correlationof the signal from the first antenna with the signal from the secondantenna. The Doppler effect is then used to determine the speed of thetarget. To do this the results from successive cross correlations arestored, and the change in relative phase between the signal from antenna1 and antenna 2 at the delay (or delays) of interest is calculated bymeans of a DFT (Discrete Fourier Transform) or FFT (Fast FourierTransform), or similar algorithm. The “delays of interest” may includeall possible delays calculated by the cross correlation.

1. A passive method of detecting an object, the method comprising receiving at a receiver a first signal transmitted by a mobile phone base station, receiving at the receiver a second signal comprising the first signal transmitted by a mobile phone base station after it has been reflected off an object; and comparing the first and second signals to derive data relating to position or speed of movement of the object; the method further comprising providing a plurality of mobile phone base stations which transmit a signal, such that as the object moves out of range of one base station, it comes into range for another, whereby the distance of the object from a base station being used to determine a position of the object remains substantially constant and distance of the object from the receiver changes, thereby improving the received power and range.
 2. A method according to claim 1, the method further comprising a time delay between receiving the first and second signals, by performing a cross correlation of the signal from the first antenna with the signal from the second antenna; determining the speed of the target using the Doppler effect; storing results from successive cross correlations, and calculating the change in relative phase between the signal from the first antenna and the second antenna at the delay of interest.
 3. A method according to claim 2, wherein the change in relative phase is calculated by means of a Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT).
 4. A passive detection system, the system comprising a receiver having first and second antennas; and processing means; wherein the first antenna is adapted to receive a signal transmitted by a mobile telephone base station; wherein the second antenna is adapted to receive the signal transmitted by the mobile telephone base station after the signal has been reflected off an object and wherein the processing means compares the signal received from the mobile telephone base station with the signal reflected from the object and derives speed or position information relating to the object therefrom; wherein the object is a moving object; and the system further comprises a plurality of mobile phone base stations which transmit a signal, such that as the object moves out of range of one base station, it comes into range for another, whereby the distance of the object from a base station being used to determine a position of the object remains substantially constant and distance of the object from the receiver changes, thereby improving the received power and range. 